Add native extensions to the Dart shell, on the linux platform.

runtime/bin/extensions.h

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+#ifndef BIN_EXTENSIONS_H_
+#define BIN_EXTENSIONS_H_
+
+#include "include/dart_api.h"
+#include "platform/globals.h"
+
+class Extensions {
+ public:
+  // TODO(whesse): Make loading extensions lazy, so dynamic library is loaded
+  // only when first native function is called.
+  // TODO(whesse): Make extension load from a relative path relative to
+  // the library it is in.  Currently loads from current working directory.
+  static Dart_Handle LoadExtension(const char* extension_url,
+                                   Dart_Handle library);
+ private:
+  DISALLOW_ALLOCATION();
+  DISALLOW_IMPLICIT_CONSTRUCTORS(Extensions);
+};
+
+#endif  // BIN_EXTENSIONS_H_